1. Field of the invention
The present invention relates to a wire dot-matrix printer employing a wire dot-matrix print head spaced from a platen by a gap selected to suit the thickness of a sheet or sheets of print paper used, and more particularly to a wire dot-matrix printer having a wire dot-matrix print head which can be moved at a speed appropriate for the thickness of a sheet or sheets of print paper used.
2. Description of the Related Art
Wire dot-matrix heads have a plurality of wires that can be driven to impinge upon sheet of print paper disposed around a platen through an inked ribbon.
In printers employing such wire dot-matrix heads, there is a prescribed gap between the tip end of the print head and the platen, the gap being hereinafter referred to as a "head-to-platen gap". If the head-to-platen gap were too small, the ink ribbon would rub against the sheet to smear the surface of the same. If the head-to-platen gap were too large, the wires would impinge upon the sheet under a reduced force, resulting in a lowered ink density of the sheet. To avoid these problems, the head-to-platen gap should appropriately be selected.
When a thicker sheet of print paper is wound around the platen, the surface of the sheet is positioned more closely to the ink ribbon, and the sheet tends to contact the ink ribbon or push the ink ribbon against the print head. With a thicker sheet employed, therefore, the head-to-platen gap is required to be increased.
Conventional arrangements for adjusting a head-to-platen gap dependent on different thicknesses of sheets of print paper are disclosed in Japanese Laid-Open Patent Publications Nos. 51-58130 and 52-106918, for example. The arrangement disclosed in the former publication achieves a desired head-to-platen gap by moving a print head into abutment against a sheet of print paper on a platen with a stepping motor, and then reversing the stepping motor for a prescribed angular interval to space the print head from the sheet of print paper. The structure shown in the latter publication includes a light-emitting element held in contact by a small force with a sheet of print paper at all times to follow the surface of the sheet, and a light-detector element disposed in a fixed position for detecting a positional variation of the light-emitting element. The print head is moved toward or away from the sheet based on a signal from the light-detector element for thereby maintaining a suitable head-to-platen gap.
Since higher-speed printing capabilities are desired nowadays, it is preferable that the tip ends of the wires be as close as possible to the sheet of print paper to reduce the interval which the wires are to traverse. In addition, the print head is required to be moved at a high speed parallel to the platen for a higher-speed printing operation.
It is customary to make adjustments for moving the print head at as high a speed as possible in the case where the thickness of a sheet or sheets on the platen is relatively small and the wires can quickly return away from the platen.
If an increased number of sheets are used for copying purposes, then the total thickness of the sheets is increased with air layers present between the sheets. When the wires hit the platen through the sheets, therefore, the wires remain in contact with the sheets for an increased period of time, and it will take a longer period of time to move the wires back away from the sheets. If the print head were moved at a speed adjusted on the basis of a relatively small sheet thickness, then the wires of the print head would tend to be caught by the sheets and be bent or broken.
The head-to-platen gap should therefore be adjusted dependent on the thickness of a sheet or sheets of print paper such that when the sheet thickness is greater than a certain thickness, then adjustments will be made to lower the speed of travel of the print head. One conventional practice, as shown in Japanese Laid-Open Patent Application No. 57-87388, has been to provide a detector for detecting a sheet thickness in excess of a prescribed thickness, independently of the mechanisms for adjusting the head-to-platen gap and the speed of movement of the print head. The detector generates a signal indicative of a sheet thickness for controlling the speed of movement of the print head so as to be lower when the sheet thickness is larger.
For higher-speed printing operation, the speed of travel of the print head has to be adjusted as well as the head-to-platen gap, as mentioned above. Although the aforesaid prior-art arrangements disclosed in the two publications can keep a desired head-to-platen gap, they fail to adjust the speed of travel of the print head in response to detection of the thickness of a sheet or sheets of print paper used.
The use of the detector for detecting a sheet thickness larger than a given thickness to adjust the speed of movement of the print head, as described above, is disadvantageous in that since the detector is disposed independently of the other components, the overall mechanism is complex and large in size.